"I think Homer gets stupider every year."—Professor Lawrence Pierce, The Simpsons 138th Episode Spectacular
The act of taking a single (often minor) action or trait of a character within a work and exaggerating it more and more over time until it completely consumes the character. Most always, the trait/action becomes completely outlandish and it becomes their defining characteristic. Sitcoms and Sitcom characters are particularly susceptible to this, as are peripheral characters in shows with long runs.
Named for one of the examples in The Simpsons, Ned Flanders, who was originally just a kind and mildly religious fellow (contrast to Homer), before becoming the obsessively pious milquetoast he is best known as today.